The present invention relates to an absolute length measuring apparatus that provides measurements of the absolute length up to a target object using laser interference.
One prevalent conventional method based on laser interference for taking measurements of absolute lengths involves combining wavelengths. The typical length measuring apparatus operating on the above method has two laser beams, say .lambda.1 and .lambda.2, oscillating alternately with wavelengths slightly different from that of the carbon dioxide gas laser(CO.sub.2 laser). The two laser beams .lambda.1 and .lambda.2 are led into a two-beam interferometer. The reflector of one arm in the interferometer is swept so that detectors's outputs are subjected to signal processing. From the detectors, there are derived synthetic wavelength signals whose wavelengths are appreciably longer than the original wavelengths of .lambda.1 and .lambda.2. The phases of the synthetic wavelength signals are measured, whereby measurements of the absolute length to the target are taken within the synthetic wavelengths.
In practice, the combination of two oscillation wavelengths from the carbon dioxide gas laser is varied appropriately. That is, measurements are taken repeatedly starting with the synthetic longest wavelength followed by progressively shorter synthetic wavelengths, until individual wavelengths are used for the measurement. The repeatedly taken measurements are processed to enhance the level of measuring precision.
One disadvantage of the above conventional apparatus is its huge size. This is partly because the apparatus uses a carbon dioxide gas laser as its light source. To generate synthetic wavelength signals in practice requires moving the sweeping reflector at least half the longest synthetic wavelength over a stage. The stage alone needs to be an elaborately constructed sizable arrangement. Another disadvantage is that it takes very long to complete the phase measurements based successively on a plurality of synthetic wavelengths as well as on individual wavelengths.